Perlite is a mineral of volcanic origin which generally falls into the rhyolitic class. A unique feature of perlite is that it contains several percent of bound water. If perlite ore is rapidly heated to a temperature on the order of 1600.degree. F. (870.degree. C.), the water is converted to steam and the perlite "pops", i.e., it expands rapidly to a much lower density. The amount of expansion is usually on the order of 4 to 20 times the original volume and the final density of the expanded perlite granules will normally be in the range of about 3.5 to 5 lbs/ft.sup.3 (0.06 to 0.08 g/cm.sup.3) for use as insulating fillers or about 7 to 15 lbs/ft.sup.3 (0.11 to 0.24 g/cm.sup.3) for plaster aggregate use. (The exact mechanism of the water-induced expansion is quite complex; see Lehmann et al., "Thermoanalytic Research on Perlite and Perlite-Type Rocks," Tonind.-Ztg., vol. 100, no. 7, pp. 271-274 (1976). For the purposes of the invention herein, however, the above simplified description of the expansion is entirely adequate.)
Perlite ore is normally expanded in large vertical chambers known as "expanders". At the bottom of each expander is a flame generating burner. The perlite ore to be expanded is dropped into the expander at a point above the flame and drops through the hot zone created by the flame. In this hot zone, the perlite particles expand and, because of their expanded volume, are then turned around and carried out of the top of the expansion chamber entrained in the exhaust gas stream. Operation of a typical perlite expander is shown in U.S. Pat. No. 2,572,484 with another variation shown in U.S. Pat. No. 2,639,132.
It has been found over the years that the perlite ore, in order to be expanded satisfactorily, must be in the form of particles or granules having a size of from about +200 mesh (greater than 74 .mu.m) up to approximately 3 to 4 mesh (approximately 1/4 inch or 6 mm). Larger particles do not expand well because their weight pulls them through the flame zone too quickly for them to be heated to the expansion temperature and in addition their greater size and stronger structure prevents heat from penetrating to the center of the particle to convert the water to steam and also prevents the steam from fully "popping" the large rigid particle. The presence of these larger particles in the perlite ore prior to expanding causes no significant problems to the perlite ore processor, for such larger materials can be readily screened out of the raw material ore stream from the expanders and recycled to crushing or other size reduction equipment to reduce the large granules to the optimum size for expansion.
At the other end of the size scale, however, the material known as "ore fines" which have particle sizes not greater than 200 mesh (not greater than 74 .mu.m) do pose a significant problem for the ore processor. These materials do not expand well because their light weight and small size causes them to be entrained in the exiting air and gas stream in an expander before they have dropped far enough to encounter the appropriate temperature for expansion. In addition, such fine materials do not move well through the ore handling systems normally in use, which commonly rely in part on gravity feed. Consequently, in the past, ore producers have considered the ore fines to be essentially waste material, although to some extent such unexpanded fine materials may find uses. Generally, however, since the largest market for perlite products is for expanded perlites, the ore processor is desirous of expanding as much of the ore raw material as possible.
It would be possible to handle such fine material if the individual fine particles were agglomerated into larger units which fall in the article size range for conventional perlite expansion. However, heretofore the known processes for agglomerating "perlite fines" have been processes intended to be used with previously expanded material. In the process of expansion, perlite (which is a somewhat brittle material) often shatters upon expansion leaving a substantial amount of fragments of expanded perlite. These fragments are themselves frequently referred to as "fines" but it must be understood that these are "expanded fines" as contrasted to the "ore fines" which are unexpanded and which are the subject of the present invention. U.S. Pat. Nos. 3,235,635 and 4,175,158 both describe processes for agglomerating the expanded perlite fines by addition of water, a fluxing agent or, in the case of U.S. Pat. No. 4,175,158, boric acid. An integral part of both of these described processes, however, is heating and maintaining the expanded perlite fines at temperatures on the order of about 1200.degree. to 1700.degree. F. (650.degree. to 925.degree. C.). At these temperatures, the fluxing materials such as soda ash or B.sub.2 O.sub.3 (from the boric oxide) act to fuse the expanded perlite fines into larger granules. Such processes are not applicable to ore fines of perlite, however, because exposing the perlite ore to such temperatures will either cause the perlite to pop (if the exposure to the temperature is rapid) or will cause the water of hydration to slowly vaporize and be removed from the ore particle (if the perlite is slowly raised to that temperature) thus leaving no water of hydration to cause the perlite subsequently to pop.
It would therefore be highly desirable to have a process whereby the fine material in perlite ore raw materials can be agglomerated at relatively low temperatures to produce a particle of the appropriate size for conventional expansion.